Distributed unit detector systems are well known. One such system is disclosed in U.S. Pat. No. 4,916,432, entitled "Smoke And Fire Detection System Communication", assigned to the assignee of the present invention.
Such systems include a plurality of spaced apart detectors which are linked by elongated, two conductor signaling cable to a control panel. The signaling cable may be long, having lengths of the order of thousands of feet. In addition, it may be desirable or necessary to connect several hundred detectors to each two conductor cable.
The detectors are conventionally coupled in parallel across the two conductors which make up the cable. Each of the detectors includes a plurality of electrical or electronic elements and energy must be supplied thereto.
One known way in which energy is supplied to such systems is to inject pulses of electrical energy into the signaling conductors at the control panel. This approach has the advantage that separate power wiring is not needed for the detectors. The only cable which needs to be run is the two wire signaling cable across which each of the detectors is coupled.
Since the primary function of the conductive cable is to provide a signaling path between the detector and the control panel, relatively small gage wire can be used. Such wire is inexpensive and it can readily be run for hundreds of feet throughout building ceiling spaces. However, such wire is not suitable for distributing substantial quantities of electrical energy.
Where number 22 or 24 twisted pair wire is used, for example, as the conductive signaling cable, it becomes very difficult to inject enough electrical energy at the control panel end of the cable, assuming it is several thousand feet long, for the purpose of powering several hundred spaced apart detectors, coupled thereto. Resistive, capacitive, and inductive losses in the twisted pair cable will reduce the voltage from the panel available at the distal end of the cable. As a result, only a small part of the electrical energy injected into the cable at the panel will be available to energize detectors at the distal end of the cable.
If the voltage at the distal end of the two-conductor cable falls too far, there will be insufficient energy to power those detectors which are located at great distances from the control panel. The problem is exacerbated if the detectors include lightable displays to provide local indicia of status or alarm conditions.
Earlier solutions to this problem have not been satisfactory. One solution has been to use larger gage wire. This increases the cost and difficulty of system installation.
Another unsatisfactory solution has been to limit the number of devices on a given two wire conductor. Another unsatisfactory solution has been to use shorter cable lengths.
A further unsatisfactory solution has been to interpose a repeater unit between the control panel and the distal end of the cable. The repeater unit serves as a power amplifier and provides, in addition to additional power, bidirectional communication between the control panel and the distal end of the cable.
As noted previously, none of the known prior solutions to this problem have been satisfactory. There continues to be a need for a cost effective and an easy to install apparatus which can be used with long, 22 and 24 gage twisted pair conductors to which several hundred spaced apart detector units might be coupled.
A preferred apparatus will be easily connected to the conductors without any provision for special junction boxes. Also, preferably, such an apparatus will be relatively inexpensive. In addition, it will be able to deliver substantial amounts of energy to the cable at locations displaced from the proximal, control panel end.